Package weakmap implements a map with weak keys. One use for such a map is storing auxiliary data with values that are not under your control. For example, if you wanted to associated additional information with values of some type *p.T, where package p is not under your control, you could use a weakmap whose keys are *p.T values and whose values are your additional information. // The advantage over using an ordinary map is that when a *p.T gets garbage-collected, your associated data is removed from the map. This should be considered a proof of concept, for educational purposes only. The problem is that the implementation requires setting a finalizer on the keys, which removes any existing finalizer. There is no way for the program to tell if there is an existing finalizer, so it cannot detect this problem.